text:
What is the maximal acceptable delay between pilot's input and flight control surface actuation?
While I was watching a cockpit video of an A330 landing in which the pilot was frenetically moving its sidestick,  I wander what was the reaction time of this flight by wire system. Indeed, the time for transmiting the signal from the sidestick to flight computer, the time for computer  to  interpret all its inputs (pilot's  input, probes,...) and to decide to act on flight control surfaces, the aircraft's reaction is not instantaneous.
Then, I realize that whatever the transmission system, there are delay between pilot's input and air control surfaces movement (material's elasticity, time for hydraulic fluid to transmit pressure, other mechanism I can't imagine).
Thus my question is: is there a maximal delay between pilot's input and flight control surface deflection to certify an aircraft?
If needed, for the FBW system, a direct law can be considered (no complex computation as flight control surface movement is proportional to input)
If needed, the question can be restricted to airliners flying under FAA and EASA jurisdictions.
EDIT: given the first feedback (comments, edits, answer), I want to highlight this question is not restricted to fly-by-wire (transmitting pilot's input through mechanical links may also induce delay)
EDIT: I think I didn't emphasize enough that this question is only about delay between pilot's input and control surface reaction. I understand that this delay is negligible compared to all other delay, but this is the one the question focus on.

It's always seemed to me that the rigid body dynamics @Jimmy mentions tend to dwarf all of the other system delays, particularly at lower airspeeds.  In other words, the control linkages could be instantaneous but you'd still see delays of as much as hundreds of milliseconds between stick movement and aircraft response, particularly at the low airspeeds reached on short final and during the flare.  That's part of why landing can be such a dance.

Peripheral: Not exactly what you are asking but of relevance to note that engine spool up times can be 6 seconds plus from throttles being "pushed to the wall". A much longer delay than would be useful in many emergency 'Go Around Power' situations.

Remember that in the Airbus, controller input does not control flight surfaces. It merely gives the flight computer a very strong hint of what the pilot wants, but the flight computer decides if the command should go to the control surfaces or not.

Very good question, and I can't give a better answer as the one already presented, but would like to add, that you should consider not only the (auto) pilot input -> processing/transmission -> effector (flight control) moves but (auto) pilot input -> processing/transmission -> effector command performed as full loop. Since e.g. engines, even though they respond to a change fast, do not reach a (thrust level) within milliseconds, rather with 1,2 seconds delay (in good conditions).

For civilian certification there are no specific requirements for certification in the FAA Part 23/25 or in the EASA CS 23/25. But obviously they require aircraft not to be prone to PIOs, even though there is no specific section addressing the issue. As @Jimmy mentioned above time delays in the control system are the main reason for type I PIOs. So designers’ objective should be minimize those time delays as much as possible.
On the other hand military requirements goes a little bit more in detail in terms of certification requirements. Aircrafts are rated as Level 1, 2, and 3 based on the time delays of 0.1, 0.2, and 0.25 seconds in the control system. Obviously, Level 1 being the best.
There is also a requirement in the same manual (Flying Qualities of Piloted Aircrafts) to define time delay in terms of phase lag. And it classifies it according to flight phases, such as takeoff and landing, cruise etc. It starts from 15 degrees and goes up to 60 degrees of phase lag for Level 1, 2, and 3 requirements.

Excessive phase lag is a direct contributor to Type I Pilot-Induced Oscillation (PIO). Phase lag comes from:

Rigid body dynamics of the aircraft (e.g. delay between elevator surface and pitch rate response)
Actuators (finite acceleration time between input and desired surface angle)
Structural compliance (e.g. cable friction)
Transport delay in signals
Finite computational bandwidth (e.g. loop closure bandwidth)

From NASA Report 4683, PIO susceptibility can be expressed assuming the pilot is compensatory; that is, the pilot input and the aircraft response would be exactly in phase, except for a constant time delay (across frequencies). This model is expressed as:
$$G(s)=\frac{K}{s}e^{-\tau_e s}$$
where $\tau_e$ is the effective time delay, or equivalently, phase rate as a function of frequency
From its research, it found that an effective time delay larger than 0.3 sec leads to PIO issues. Given a typical pilot time delay of 0.2 sec, this would imply an upper bound aircraft effective time delay of 0.1 sec at higher frequency (around 5 rad/s), end to end.

The exact time depends on flight speed. Slow flight will tolerate a larger delay since aerodynamic eigenfrequencies go up with speed. At a higher frequency the same delay means more phase lag.

@PeterKämpf MIL-F-8785C specifies the desired eigenfrequencies for the rigid-body modes. With the exception of SP, they are speed-independent (but flight phase dependent). The NASA paper went through both fighters and transport category aircraft; it's pretty wide ranging.

That must have been poor research. Human reaction delay is well studied and on the order of a quarter second. With training and good health it may be less, but never below 0.1 seconds.

@PeterKämpf We can narrow down the effective vehicle time delay further by including pilot delay. Amended my answer.

This is a classic problem in control system theory. The condition to be avoided at all costs is the case where the pilot's control actions get out of phase with the movements of the plane, so the sidestick-action makes the oscillations worse instead of damping them out.
The two ways that could happen are 1) if there are significant processing time delays in the control system connected to the sidestick and 2) if there are significant delays in the pilot's reactions.
As pointed out above, the control system time lags are tiny compared to the time constants of the plane's responses to aileron movement, etc. and the significant time lag in the overall system consisting of plane + pilot + computer control system is in the PILOT, not the control system.
This gives rise to something called PIO or pilot-induced oscillation, where the response time lag of the pilot pushes the whole system into divergent oscillation- as for example in the case of a pilot porpoising a plane down the runway after bouncing off the runway on his or her initial touchdown.
I do not know if computerized flight control systems contain subroutines that prevent PIO but perhaps Peter Kaempf knows!

answer has been corrected- NN

The best way to prevent PIO is to take the pilot's reaction time out of the loop. The second best is to keep the pilot sober and not tired to shorten that delay. I don't have access to FCS source code but have a difficult time to imagine how else to compensate for the delay from human reaction time.

Good answer but rather than “As pointed out above...” it’s best to refer to the specific answer, even better link to it. As the voting system may mean the answer you’re referring to isn’t always above yours.

@PeterKämpf: One way in which a skilled pilot can prevent PIO is by noticing when it's occurring and taking action to de-excite it — such as (seemingly paradoxically) by deliberately slowing down their control responses and/or reducing their amplitude in order move them out of sync with the oscillatory frequency and to reduce the loop gain below unity. Or in human terms, "stay calm, keep steady, take it slow and easy." In principle, an FCS could also actively detect PIO and adjust the control response to de-excite it — although in practice, the best option may be simply to alert the pilot.

@IlmariKaronen: Correct – in many cases it would had been best to just keep the stick centered. But try to realize what is going on and have the nerve to respond accordingly in a stressful situation – I cannot blame pilots who were trapped in a PIO. Your desire is to get out of it, so you apply what you have learned. Even if it makes matters worse. The insight comes much later.

The Airbus normal flight law does try to get the pilot out of the loop as much as possible while staying under their control. The stick position corresponds to desired pitch and roll rates, so when the pilot lets go of the stick, the system will actively stop the motion, avoiding the need for the pilot to command opposite deflection and possibly starting the oscillation.

There is quite some experience in this in Level D simulators, which have computer generated responses that must match those of the original aircraft, within tight tolerances.
A couple of decades ago, the gold standard for Unix real time host computers was 30 Hz. So 30 times per second, all of the following was computed:

Surface deflection from stick input, including cable stretch, oil flow simulation etc.
Aerodynamic hinge moments on the surface.
Hydraulic hinge moments exerted by the actuators.
Aerodynamic forces amd moments on the aeroplane.
Inertial response of the aeroplane.
Visual system response.
Motion system response.
All other system states and responses.

With an update rate of 30 Hz the standard was deemed acceptable for Level D zero flight time training, which implies a time delay of 1 frame = 0.0333 sec. So we know that this is fast enough: frequency rate 30 Hz, time delay 0.0333 sec.
As an aside, for present day computers this iteration rate is something to smile at, the code that ran @ 30Hz on a state of the art realtime unix machine runs @ 3000Hz on a Macbook Pro now.

Computation time isn't the only time delay. More pronounced time delays include transport delay in signals and confirmation delays (should they exist).

@Jimmy Indeed. 30 Hz in the simulator computer was fast enough to include all signal and other delays that occur in the aeroplane though.

@Jimmy As Niels points out, the largest delay is the pilot response.

Is there a maximal delay between pilot's input and flight control surface deflection to certify an aircraft?

Literally, no.  The FAA's only pronouncements about latency are about ADS-B.
To measure what you're asking about, a temporal delay is too simplistic.  You need something like the system's band-limited impulse response, or its temporal equivalent of a modulation transfer function.  And not just from stick deflection to surface deflection, but all the way to rate of change of (say) roll rate.  FAA doesn't even try to enforce numbers on the output of that process, never mind the intricacies leading up to it.
If an aircraft's control latency in some respect was dangerously large, the test pilots (or the flight simulators!) would notice it well before certification forms were sent to the FAA.

In the early 1980’s, the Flight Research Department at Arvin/Calspan ATC investigated the issues associated with the digital flight control systems, commonly known as Fly-By-Wire (FBW), which were becoming more common in aircraft such as the F-16, F-18, and Tornado. In that day, computer processing power was a mere fraction of what is available today, and the problem was compounded by complex control systems. The researchers were able to induce latency into the flight control system which was in addition to the inherent latency of the test aircraft, and have test pilots evaulate controllability issues in high stress maneuvers such as landing and target tracking. Their results were published in a paper entitled “Effect of Control System Delays on Fighter Flying Qualities”, papers presented at the Flight Mechanics Panel Symposium on Criteria for Handling Qualities of Military Aircraft held in Fort Worth, US, 19-22 April 1982   AGARD-CP-333 (Advisory Group for Aerospace Research and Development) AD A118596
In brief, what they found was at beyond to 130 msec delay in longitudinal control, and 120 msec in lateral control, pilot perception of controllability was reduced. For every 25 msec extra, the Cooper-Harper Pilot Rating score (1-10) increased by 1.
In flight simulator, we have to ensure that the INDUCED delay in the simulation in not more than the Specification. For military aircraft, that is generally 100 msec for fighter and helicopters, and 150 msec for transport aircraft. The FAA has looser standards, 1550 to 300 msec, depending on the level of the simulator.

The technical term used is latency i.e. the propagation (or transport) delay between the input (pilot control) and the output (control surface movement). The aircraft designer (or Original Equipment Manufacturer) determines the acceptable latency.
The acceptable latency depends on the type of aircraft i.e. Airlines, General Aviation, or Hobby aircraft, flight control dynamics of the particular aircraft, the systems through which the resultant signal is produced (Pilot Control Sensors -> Flight Control Computer/Mechanical Linkages -> Actuation Unit -> Surface movement),  and the critically of the signal (ex: control surface actuation).
For airlines like Airbus (A330), or Boeing (B787), the latency between the pilot control inputs and the flight control surface actuation usually ranges between 50 to 100 msec.

Do you have any docs for further readings (I'm curious as you present a latency than can double from one case to another, and how it is decomposed between the different steps you describe) ?

The design requirement document of the particular aircraft will contain the latency requirements and this document cannot be disclosed. As specified in the answer the acceptable latency depends on the various aspects, and it is different for different aircraft. Therefore, it is mentioned in a range, and it is not quite big from the complete propagation delay perspective, even though it is double.


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What was the first aircraft equipped with a RAT?
RAT are common in airliners today to provide  emergency electrical and hydraulic power. Yet, firsts airliners (prior to WWII) may not require the same amount of power than today's airliners (which are bigger and require electrical and hydraulic power to keep all electric-assist requirements, computers, hydraulic actuators,...).
The historic section of the wikipedia article speak about windmilling on the FW-189's engine as a precursor to RAT, but I fail to see anything related to first device whose only role is provide power.
My question is: What was the first aircraft equipped with a RAT? Was it military or civilian (it may not be a airliner)?
EDIT: given the comment by Peter Kämpf, I edited the question to include all aircrafts. Thus having an answer mentioning both airplanes and other aircrafts.

This being about electrics and first use, my entry would be a Zeppelin. But you are asking about airplanes, so lighter than air is not included. In the question text you say aircraft, so maybe a Zeppelin would qualify? Please clarify!

@PeterKämpf edited accordingly

So far, the oldest I have found is LZ-127 Graf Zeppelin, first flight in 1928. It used a propeller-driven generator to power its radios and a second one (with buffer battery) to keep the ship illuminated. In the drawing below (source) they are indicated as "Wind-Dynamo", being on both sides of the main gondola.
I am rather sure that the wartime Zeppelins used those generators as well, but have yet to find proof for it.

LZ-127 on August 26, 1929, in Los Angeles (source). The right generator is circled in red.
EDIT: Now I have found a source that attributes a RAT to the Zeppelin Staaken Riesenflugzeuge of WW I. The source is Tom Morrison's book "Quest for All-Weather Flight" and it says on page 28:

In November 1917 the Drexler bank indicator was introduced instead [of the Anschütz gyro]. Much simpler than the Anschütz instrument, it weighed only 5 kg (11 pounds). It was powered by electricity from a generator driven by a small propeller in the slipstream and took less than a minute to run up to speed.

So this dates the first RAT back to November 1917. Sorry, no online sources.
ANOTHER EDIT: I could have thought of that earlier. Really! Igor Sikorsky illuminated the cabin of his Ilya Muromets four-engined giant airplane using light bulbs, powered by a wind-driven generator, too. Wikipedia cites a Sikorsky source from 1938 as:

Lighting was provided by a wind-driven generator and heating was supplied by two long engine exhaust pipes which passed through the corners of the cabin.

Since the earlier S-21 Russky Vityaz had already a similar cabin, it is likely that the first RAT was used as early as 1913. But the first source I found so far firmly establishes it in 1914 if not earlier, in a civilian passenger airplane later used as a bomber. Not an airliner, though, because those came later.

Russky Vityaz. Clearly visible is the electric searchlight in the open fuselage nose stand and less visible (circled in red) the wind-driven generator used to power it. At least that is what I think it is.
Wikipedia says about the legal status of this picture: This work is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before January 1, 1926

Even the very first aircraft that were retrofitted with electric instruments had air-driven generators (and Venturi tubes for gyroscopic instruments). Engine-driven generators and vacuum pumps came later.
Here's an account of the first non-stop transatlantic flight in June 1919, which suggests air-driven generators were pretty much standard by then. From Wikipedia:

It was not an easy flight. The overloaded aircraft had difficulty taking off the rough field and only barely missed the tops of the trees. At 17:20 the wind-driven electrical generator failed, depriving them of radio contact, their intercom and heating.

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Is there an equivalent of cash transportation in air cargo?
Some cargo (cash for banks, precious art pieces, gold, jewellery,...) are transported in bullet proof trucks with armed guards while travelling by road. Sometimes it may also be escorted by police.
Is there an equivalent for air transport? How is protecting handle for such cargo when transported by air (both at the airport (load breaking and ground operations) and in flight)?

Comment since it doesn't really answer the question, but if you look at the actual thefts that have been carried out on high-value cargo it's always done at the loading/unloading stage, eg https://en.wikipedia.org/wiki/Brussels_Airport_diamond_heist or https://en.wikipedia.org/wiki/Lufthansa_heist

Decades ago, when we moved checks at night for Federal Reserve clearing, we would sometimes have sealed bags which were cash.  On my flights, I never had an armed guard on board, but on taking off and landing, we were met on the ramp by an armored truck with armed guards on a routine basis.
The impression I had is that checks and other paper were the main cargo, and that cash and bullion were minor incidentals.
Related, items of national security are routinely carried by scheduled airliners, and are placed into the hold by cleared individuals, and then the hold is opened upon arrival, in the presence of cleared individuals who retrieve the packages.  Contents are logged, in the event of compromise.  Similar parcels are also handled by the major parcel and courier enterprises, with slightly different processes.

Gold ingots are frequently transported by airlines. One shipment I handled was ingots the size of a VHS tape, it was wrapped in a tough thick paper somewhat like tar-paper and had steel banding. It was surprisingly heavy and caught me by surprise as It tried to pick it up by holding the edge and it wouldn't move. I had to slide it to the edge of the trolley before I could get my fingers under it to lift it.
Most other times gold would be shipped in heavy wooden crates but one shipment of gold was packed into bright blue 50gal drums.
Around 2004 Malaysia launched the polymer 5Ringgit note. This travelled from Australia to Malaysia in the belly of Malaysia Airlines 747 aircraft.

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How is rudder used in a aileron roll?
Rudder is usually used to maintain coordinated flight. When banking to one side to begin a turn, the rudder is usually pushed towards that side. I guess this is still valid in inverted flight: in normal flight, if the pilot bank the plane to the right, it also push the right pedal (to conteract adverse yaw); in inverted flight, if the pilot bank to the right, the plane will turn to the right seen from the ground, i.e. the left seen from the pilot. The pilot should apply left rudder to keep coordinated flight.
If the pilot want to make a aileron roll, what will be the general action on rudder given the flight goes quickly from normal to inverted and back to normal, and the pilot don't want to initiate a turn?

The key in flying a good roll is to keep the aircraft's nose up in anticipation of the lift requirement when flying in knife-edge or inverted flight. Since in knife-edge the elevator and rudder will assume each others function, you will need rudder as well as elevator for directional and pitch control. Also, depending on adverse yaw, some rudder is required to hold yaw to zero when applying ailerons. The procedure is as follows:

Speed up, so you are fast enough for roll maneuvering and inverted flight.
Pitch up a few degrees. How much depends on wing incidence and zero-lift angle of attack. Look out for some feature on the horizon: This will help you to keep your direction constant.
Stop the pitching motion, then apply full ailerons. Correct the aileron-induced yawing motion by applying gentle rudder (trailing edge in the direction of the up-going aileron). Your goal is to keep the fuselage pointing at the chosen feature throughout the maneuver.
With increasing roll angle, apply rudder to keep the nose up. At the same time, push the stick gently to avoid course deviation. At 90° roll angle, both elevator and rudder have changed their function: The elevator will now control direction, and the rudder will control pitch.
Keep the rotation rate and push the stick more when changing from 90° to 180° roll angle. In inverted flight you need to keep the aircraft trimmed, and depending on the static stability and speed this can require considerable negative elevator deflections. At the same time reduce rudder deflection such that the fuselage will still point into the initial direction. At 180° roll angle both elevator and rudder have returned to their old function, but pulling will now start a dive.
On the way to 270° you will again need to add some rudder, now in the opposite direction, to keep the nose up. Again, elevator and rudder will exchange their functions.
Once you return back to normal flight attitude, stop the rolling motion and reduce rudder deflection back to zero.

Wow, that's a lot of work!

@FreeMan: It becomes less if you fly an aircraft with symmetric airfoil and zero incidence. An aerobatic aircraft, in other words.

Also flying is not too different from driving.  Bet you can parallel park without thinking too hard about it.

Aileron rolls are really easy, even if they do have 7 steps!  Falling Leaf and Dutch Rolls are great coordination exercises for the loop.

@rbp your comment just make me even more curious about aerobatics. I may ask other questions in few days (after reading about falling leaf and other aerobatics yet unknown to me)

Well first, by barrel roll do you mean a true barrel roll which is a combination loop and roll, or do you mean a tighter maneuver like an aileron roll or slow roll accomplished mainly with ailerons but which can still result in a rotation around a point above the plane?
The answer in all three cases is that rudder is used to help maintain stability in the roll. A little rudder at just the right time can help keep the nose up in a slow roll or even an aileron roll, as the plane rotates through the sideways portions of the roll when gravity is otherwise not countered. Counter-rudder (applying rudder opposite to a bank) can also help to start a "wide" roll by kicking the plane into an initial yaw that counters the tendency of the plane to turn into the initial bank. Finally, first a hard left yaw and then a hard right yaw is used to execute a "true" barrel roll where the plane rolls in a circle roughly perpendicular to its original path.

I meant aileron roll. Question edited accordingly.

As an aerobatic pilot, it's pretty simple.  The aircraft will lose lift as you roll through 90 and 270 degrees - knife edge flight. Obviously this will cause loss of normal level flight pitch. So, you have to compensate for that pitch loss you start by adding the pitch you will lose (depending on speed and roll rate of the aircraft) as you start the roll. How much? Start by fairly quickly pitching up about 40 degrees and adjust from there each time you do a roll until you start and end at the same altitude. Rudder? If you roll left, slowly add right rudder topping out at 90 degrees. Reduce rudder to zero at 180 degrees and push the stick gently forward to zero g's. If you go negative, reduce pressure so you float gently through 180 degrees. Reduce that forward pressure to neutral as you come through 270 degrees and simultaneously add left rudder. At 270 degrees continue reducing left rudder to neutral as you come to level flight.  Once you master a left roll, just reverse rudder procedure for a right roll. Be gentle with the plane as this should be done with very little stress on the airframe... if it does, you are doing it wrong. Learn how on a plane certified for aerobatics before you try it on the company A380.

There are 2 types of rolling maneuvers used in aerobatic competition, a slow roll and an aileron roll. First the easy one, an aileron roll.  Simply raise the nose above the horizon, apply slight rudder in direction of roll initially to counter adverse yaw, stick in direction of roll. You can do multiple rolls, the limitation being that you must stop once the nose has fallen about 30 degrees below the horizon. A beginner can accomplish this maneuver with minimal instruction.
The slow roll has nothing to do with the rate of roll, a high level aerobatic plane can slow roll or aileron roll at 420 degrees per second. The slow roll requires a bit of coordination in that the plane rolls about it's longitudinal axis, pitch not changing during the maneuver, as stated previously, rudder is used at 90 and 270 degrees, and forward elevator at inverted, to keep the plane at constant pitch and the nose not yawing left or right.  Of course no one should ever try to teach themselves aerobatics. Get a good instructor and a good aerobatic plane.

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What is MCAS trying to fix on B737 MAX?
New sources such as Seattle Times and Aviation Week provide a pretty good overlay of the MCAS function on B737 MAX. However, I'm unclear as to the issue that MCAS is attempting to fix. From what I can gather, the larger nacelle led to a shortening of the pylon height, leading to altered aerodynamics at the wing/pylon/nacelle junction. At high AOA, this leads to a higher pitch-up moment than on the NG (dispelling other popular theories, it has nothing to do with the thrust line or nacelle CG).
In Seattle Times, it talks about shock formation at the junction, indicating that this is not a low Mach problem. However, transonic regime is usually not limited by stall but by deterrent buffet. Can someone with the knowledge clarify? If it is a stall issue, is it a lack of stall identification (lack of distinct pitch down) or is the aircraft experiencing a pitch-up without MCAS?

All airplanes stalls. Most airplanes only stalls when the pilot instructs it to do so. The problem of Max is, it could stall itself, slowly. MACS is added to correct itself from doing so.

@user3528438 How is that different than a 737NG?

The Ethiopia and Indonesia crashes did not happen slowly!

Closely related: https://aviation.stackexchange.com/questions/61910/did-a320-neo-need-an-equivalent-to-737-max-mcas-because-of-its-bigger-engines?rq=1

@Harper They didn't crash from a stall, they crashed when the MACS tries to correct a false stall, and it did happen across several minutes.

From The inside story of MCAS: How Boeing’s 737 MAX system gained power and lost safeguards by Dominic Gates and Mike Baker:

During flight tests to certify an airplane, pilots must safely fly an
extreme maneuver, a banked spiral called a wind-up turn that brings
the plane through a stall. While passengers would likely never
experience the maneuver on a normal commercial flight, it could occur
if pilots for some reason needed to execute a steep banking turn.
Engineers determined that on the MAX, the force the pilots feel in the
control column as they execute this maneuver would not smoothly and
continuously increase. Pilots who pull back forcefully on the column —
sometimes called the stick — might suddenly feel a slackening of
resistance. An FAA rule requires that the plane handle with smoothly
changing stick forces.
The lack of smooth feel was caused by the jet’s tendency to pitch up,
influenced by shock waves that form over the wing at high speeds and
the extra lift surface provided by the pods around the MAX's engines,
which are bigger and farther forward on the wing than on previous
737s.
(...)
Under the proposal, MCAS would trigger in narrow circumstances. It was
designed "to address potentially unacceptable nose-up pitching moment
at high angles of attack at high airspeeds," Boeing told the FAA in a
proprietary System Safety Assessment reviewed by The Times.

Another article from The Seattle Times published in March makes mention of a high speed stall:

Designed to activate automatically only in the extreme flight situation of a high-speed stall, this extra kick downward of the nose would make the plane feel the same to a pilot as the older-model 737s.

This indeed points to a stick-force-per-g test, as @Jimmy mentions in a comment. Since the B737 has a fully irreversible hydraulically actuated flight control system, with an artificial feel that is proportional to dynamic pressure but not to load factor, the obvious mechanism for "lack of smooth force feel" would be having to release the column to some extent due to extra nose-up moment that the aircraft generates.
It is very plausible that the added lifting surfaces from the engine pods create a larger pitch-up moment than the 737NG has in the same circumstances. I'm not really sure how the supercritical airflow over the MAX wing differs from that of the NG to create extra pitch-up.
All of that is the original limited cure for the stick-force-per-g tests. If this was the only thing that needed to be fixed, all would probably have remained well. But the first linked article also mentions:

The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions. To cover that issue too, engineers decided to expand the scope and power of MCAS.

As you can see, The Seattle Times is my source of information as well...there is no additional information on what were the "certain low-speed flight conditions".

So this sounds like a buffet penetration issue more than a stall issue?

The way I understand it, the forward mounting causes an extra pitch-up moment in a particular situation: high AoA, high airspeed, high bank angle. Flow separation would seem a more likely cause for this than buffeting.

The fact that it is a WUT means it's a stick-force-per-G test. Coupled with reference to high speed and shock formation (assuming the same author got it right), it sounds like a maneuvering characteristic (i.e. stick force per G) issue at high Mach, rather than a stall issue. Whether the uncertifiable characteristic occurs prior to buffet onset or during buffet penetration is not clear. I guess that's where the name MCAS comes from!

Indeed, that is the way I understood the original design purpose of MCAS as well, although the original article I read in March makes mention of a high speed stall. The SeatlleTimes article you refer to indeed mentions shock waves and the increased lifting surface from the pods in front of the wing.

Thanks for the additional sources. I don't think that the "lack of smooth feel" comes from a servo limitation or the Mach trim. It's more likely that it refers to stick force lightening (uncertifiable prior to buffet onset) or reversal (uncertifiable up to deterrent buffet). Mach trim wouldn't have helped since this is maneuvering stability rather than speed stability.

Yes indeed, reversal is what I was describing, the actuators not being able to overcome the aerodynamic hinge moment.

What's the evidence suggesting that the actuator reached hinge moment limitation? Stick force reversal can occur when actuators are well within HM capability. The explanation of nacelle induced shock and the resulting pitch-up seem to jive with the news sources and reported symptoms.

How does stick force reversal work with a fully irreversible actuation system?

It's no different than in a reversible. During wind up turn, you'll have to reverse the stick force to maintain the load factor at constant speed.

Few people and even fewer journalists know what a high speed stall is. The fact that the pitch-up tendency is strongest with high thrust makes clear that accelerated flow over the intake lips at high angle of attack contributes the final drop in the bucket that was too weak with the smaller, more rearward engines. Forget shocks - they usually shift the center of pressure back.

Was there a problem with stalling on 737 jets before the 737 Max,

The flight characteristics, including high angle of attack operations, of the previous models of the 737 did not warrant the kind of action the MCAS system provides.

why is MCAS needed on the 737 Max 8

The Max 8 engines are physically bigger—69.4 inches fan diameter versus 61.0—heavier, mounted farther forward and higher than previously. This changed the thrust line and flight characteristics. MCAS is used to restore "normal" 737 flight characteristics.
See http://www.b737.org.uk/737maxdiffs.htm for details.

Would you agree with the phrase that was added to your answer without any reference? "In particular, the new engine cowlings added enough forward lifting area to affect the plane's handling at high pitch angles."

@Koyovis To the best of my knowledge, I didn't post this answer even though it's attributed to me. I say to the best of my knowledge because I am subject to some dementia, and I've learned I can't always trust my memory anymore. However, I've never flown any model of the 737 and really know nothing of it. My large aircraft were the 727-100 and 747-100/200. I looked at the link in the answer, and I'm as certain as I can be that I had never seen that web page before. I don't know what's going on.

If I click on the "edited.." link underneath the answer, I can see your original post of April 2019, plus a phrase added recently by @GuyInchbald. I wonder if you agree with that phrase, or if you would prefer to roll back the edit. If the latter, we can do that for you if you prefer.

I added link and quote to explain the nacelle effect on stability, don't know if b737.org.uk is considered an authoritative soure though...

@Jpe61 Please respect the original poster of the answer. I would never fundamentally change or add to an answer of another user here, but would post my  own answer if I reckoned more details are required.. Now Terry's name and face is underneath some text that he never published..

@Koyovis, sure thing, rolled back. I do not understand how this improves the answer though.  The recently added edit by Guy should be removed too.

@Jpe61 Agreed, did so.

My apologies for my faux pas but I am relatively new to StackExchange. I see now that I was summarising a significant quotation that had been added then deleted some time before. OK, I'll post it as a separate answer.

Please accept my apologies for my faux pas, I am relatively new to StackExchange. I now see that I was summarising a longer addition that had previously been made and removed, and have posted a separate answer.

@GuyInchbald No worries.

Previous 737 models had acceptable stall characteristics.
The Max introduced new and larger engines. There was not enough room for them between the wing and the ground, so they were moved forwards further in front of the wing to make room for them to sit high enough and clear the ground. This affected the handling of the Max at high pitch angles.
But the intention was to introduce the Max without the need for significant pilot re-training and type re-certification, so the change in handling was unacceptable. MCAS - the Manoeuvring Augmentation Control System - was introduced to manage the aircraft handling at high pitch angles and avoid the need for pilot re-training.


text:
How to calculate the forces and moments required to aerodynamically actuate slats?
I am trying to make a wing where slats automatically deploy once it hits a higher angle of attack, let's say right before stall.
I need to roughly estimate the forces and moments acting on the undeployed slat.
Here is a sample airfoil with the pressure distribution superimposed:

Source: own work
I plan to operate at a Reynolds number of 100,000 at stall speed, and the preliminary dimensions for the slat position are 10% for the width, 1.5% for the depth and 2% for the gap, all in chord percentage. Refere to the image below for the definition of the dimensions:

Source: Effect of Slot Span on Wing Performance, J.Granizo
I think the next step is to use the following equations for the slat alone:

Source: Fundamentals of Aerodynamics 6th edition
However, I only know the pressure and friction coefficients on the top surface (marked in red line). Do I assume that the entire bottom surface (marked in blue line) is at atmospheric/freestream pressure?
A point in the right direction would be appreciated.

This doesn't sound very simple. What will be the distance of the slat from the airfoil? What is your Reynolds number? How the slat will be moving? It would be nice to put more details to your problem for people to point you in the right direction. The above equations are just a high-level force analysis on the slats assuming you know the $\Delta C_p$, $c_f$ etc.

I have provided more information in the post. The plan is to have the slats held by a cantilever snap-fit joint inside the main airfoil (like a pen cap). I want to know if the suction force is strong enough to pull the slat off of the joint, which I measured to be around 2N of force

As a pilot, I don't want anything uncommanded going on with wing surfaces. Maybe have a pressure sensor that lights to indicate it might be a good idea to deploy the slats?  There are some light aircraft that have permanently mounted slats like you drew.   Here is an article the Zenith  STOL CH 701 that uses them  http://www.zenithair.com/stolch801/design/slats-vs-vg-design.html

Very interesting challenge and very interesting design approach.
I've read this had been done few times with TE flaps but not with LE slats. If i were you I would take the following approach to figure a way out.

In Javafoil you can model open slat multi element air-foil. The results are not very accurate at your Reynolds number if there is a huge separation bubble though. xfoil does not support this feature at the moment but if you are thinking of using TE Flap your best bet is to use xfoil). Calculate several slat gaps by geometrically altering the input file for a list of angles of attack.
You can easily export the local CPx data to a text file in Javafoil. Then use these data to find out how much of aerodynamic force the slat generates by integrating the local surface slope and CPx values for each slat-gap, alpha pair.
Then this data-set could be interpolated to find out what would be the slat-gap distance for given angle of attack for a given spring constant and substantially to find L/D for each angle of attack.

Hope this helps.
ABCD
PS: One example of automatic TE flap deflection is discussed here. I think LE slat would be much more effective because $Cp_\alpha$ near the LE is much higher than that of the TE

Your example of "automatic TE flap deflection" involves a high speed launch, but for a smaller model might work with rubber bands.  Spring loaded leading edge slats have been around for a while.

Curious as to the application and how the slats would extend (springs, electric motor, actuated by pressure sensor). What is most interesting is the choice of wing.  Thin undercambered wings, which could be considered a wing with slat and flap already deployed, have amazing low speed lift performance, but generate so much lift at higher speeds that they become impractical.
If one studies wing evolution in aircraft wings, thin under camber gave way to flat bottom, then semisymmetrical, then thinner as speed continually increased.  Modern airliners are perfect examples of slow to fast every flight.  They reconfigure for LESS lift and drag so they can speed up and fly efficiently at cruise speed.  But an 800,000 lb 747 dropping to 1/3 of cruise speed (1/9 lift!) and reconfiguring to the days of Bleriot with (Fokker Triplane) Fowler flaps is an engineering marvel indeed.
Modern "slipper" glider wings are not nearly as undercambered.  So what will this plane be?
So, as far as the right direction, I would look into
air foil tools on the Net and check out some existing designs for your application.
If you want to slat the wing you have, spring loaded automatically deploying (probably by lower air pressure) slats have been used on fighter planes such as the Sabre jet. But the wing you have will be very good for slow flight as it is.

It's for a design challenge in which the goal is to get the highest score - which is to maximize the area under the L/D vs AOA curve from 0 to 20 deg, normalized with the airfoil's cross-sectional area. So the best score will be obtained by having a good L/D performance for a wide range of AOA & lowest wing volume. I figured I can try to increase the L/D at higher AOA with slats to improve my score because they will be using the cross-sectional area of the airfoil w/ undeployed slats, and wanted to ask if the suction will be strong enough (2N of force) that I can get away w/ not using springs.

Chord length is capped at 10cm, so it will be hard to find small diameter springs to fit inside the main airfoil.

Ok!  Lift of this type of wing will start at negative AOA.  If unable to make it retractable, maybe fixed slat (slot) like Fiesler Storch and birds! Good luck.


text:
Is it possible to estimate the location of aerodynamic center without experimental moment data?
In airfoils, the aerodynamic center is usually found close to the quarter chord point, but for quick calculations, we assume it lies on the quarter chord.
What I am looking for, is if there is any way to find a rough estimate as to where the aerodynamic center is actually located on a tested airfoil - given only lift and drag data.
The tested airfoil is mounted on the sting on its quarter-chord point, which sits on top of a force transducer. The angle of attack is varied throughout the test.

The pressure distribution $c_p$ on an airfoil is necessary to determine the aerodynamic center of an airfoil. From the pressure distribution you can obtain all three coefficients $c_l,c_d,c_m$ (well, for accurate $c_d$ you better have a rake for high $L/D$ airfoils). Having only the first two, as in your case, is not enough. Simply because there are many $c_p$ distributions that will give the same $c_l$, $c_d$ forces and different $c_m$ forces. These airfoils will also have a different aerodynamic center. Thus, the answer is no. If you don't have means of measuring the moment, couldn't you just vary the mounting position of the airfoil to estimate moments and do your job?
However, in case you want only a rough (experimental) $c_m$ estimate you can relate your airfoil basic characteristics ($c_l,c_d$, zero lift angle, camber and thickness distributions etc.) to similar airfoils found in this classic reference and interpolate.


text:
Can a fighter jet be stolen?
What (if any) sort of pilot authentication systems do fighter jets have?
Could anyone with the required skills simply take the aircraft or is a key or authorization code required?

This question brought to you by screenwriters of the next xXx franchise film.

Related: http://aviation.stackexchange.com/questions/9005/what-stops-planes-from-being-stolen

Based on my extensive research, I'd say that it looks quite hard.

You really think you want pilots looking for keys when they have to scramble?

Planes, tanks, trains, trams, buses - they usually don't have any keys. If you can get in and know how to operate one, you can do it. Large organizations operating them tend to rely on procedures, not on mechanisms.

@Agent_L, and heavily-armed guards, backed up by radar networks, surface-to-air missiles, other fighter jets to pursue, a limited number of places to land...

Regarding the latest edit...that's not a spelling error, that's the English (proper) way to spell it ;)

@Notts90 uh oh, here comes a controversy. Honestly, as a canadian, I have no idea where I'm supposd to stand on this one.
As a programmer though, US spellings have been beaten into my head.

@Notts90 Also, kind of annoying that my minor edit covered up your name, since you're responsible for most of the phrasing and appropriateness of the question

@dn3s See this question :-)

@dn3s no worries, just pointing out it's not an "error".

When I saw this on the HNQ, I thought this was an Arqade question.

Most fighter jets are protected by large numbers of people with guns that don't want you to take the plane.

I guess I'm a little disappointed by the emphasis a lot of comments/answers are placing on external protections. It's really not new information that military equipment on a military base will be guarded by military personnel.
I mean, I have nothing against witty quips, but it seems people actually think someone might be unaware that fighter jets are kept under armed guard.

This is how you steal a fighter jet (in movies, at least). BTW they have some of the best aerial shots.

@Notts90 The French way, you mean :)

When you're Clint Eastwood, stealing a fighter jet is actually quite easy. It's getting there that takes all the work.

As Omegacron mentions, one option to avoid the physical security is to disguise yourself as the official pilot, a la Clint Eastwood in Firefox.

@Omegacron Provided you can think in Russian.

@dn3s: You asked what pilot authentication system fighter jets use. People are answering that the pilot authentication system consists of the military forces surrounding the aircraft, rather than anything internal. People are emphasizing the external protections not because they think you didn't realize the external protections exist, but because you weren't aware that the external protections are the answer to your question.

@DrMcCleod - Da, et vazhno.

@dn3s We're sorry that the answer to your quesiton is more boring than you were hoping for. Unfortunately, the answer is the answer.

@DavidRicherby sorry, my comment came off more bitter than I meant. I do think we have some good answers here!

I don't know, ask him https://aviation.stackexchange.com/users/16965/fighterjet

That spelling modification upsets me.  It's not a correction.  There are far more countries in which the -s- spelling is standard, than where the -z- spelling is standard.  English was the language of England long before it was spoken in USA.  You can kind of tell that from the name.  There is no earthly reason for Americans to impose their spelling norms on the rest of the English-speaking world.  Please, can we nip this kind of anti-correction in the bud?

@DavidWallace http://blog.oxforddictionaries.com/2011/03/ize-or-ise/

Yes, I'm aware.  It doesn't justify changing someone else's perfectly correct spelling.

i made the edit because it was originally my question, with my name under it, and it isn't how I would spell the word. It's not like having a question with my name on it with "authorisation" is some unbearable embarassment, but it's very low cost to change it so why not? It's just an (extremely minor) way of expressing (an extremely peripheral aspect of) myself.

Comments are not for extended discussion; this conversation has been moved to chat.

Military aircraft rarely have any sort of keys or any sort of authentication systems, they rely on physical security measures to prevent people from getting to the airplanes in the first place, like fences, dogs, cameras, guards, and of course guns.
If there were no physical barriers then you have 3 major barriers to being able to steal a fighter jet:

Support: just about every military fighter aircraft has an assigned crew to prepare it for flight and help the pilot(s) get in it. Without this crew you'd have to get the airplane powered up, fueled up, and then you'd have to figure out how to get into it and get the ladder removed. There's a lot of knowledge you'd need to do all this, just knowing how to open the canopy isn't going to be immediately obvious
Starting: starting up a fighter isn't like starting a car, you don't hit a button and everything happens for you, there's fuel tanks and pumps, hydraulic systems, electrical systems, and engine management. You'd have to know where to find all of the switches, dials and levers, and the sequence in which to use them. This isn't something you'd be able to guess, you'd need knowledge of the particular airplane
Flying: high performance jets are challenging to fly, and by flying I mean aviating, navigating, and communicating. Although modern fighters have a lot of computers to help out with this the systems require a great deal of training to use, and if you don't know exactly what you are doing you are most likely going to dig a big hole in the ground

If you know how to start and fly the airplane there's nothing stopping you from doing it, provided you can solve the problems of prepping and getting in the airplane. In fact, it's been done when pilots defect with their airplanes, see the comments below for some great examples.

That said, fighter jets are occasionally "stolen", most typically by fighter pilots who defect from their country of origin using said fighter jet as their means of escape.  This usually happens during what should otherwise be a normal training exercise, which takes care of the issues of getting access to, starting, and taking off with the aircraft in the first instance.

Slight quibble with #3; many fighters are supposedly quite easy to fly if you fly them straight and level as you would a 737.  The difficult part is flying them under combat conditions and using their capabilities effectively, which could be needed if you stole one and were detected by the owners.

@J... This is my favorite example of a pilot defecting and stealing an airplane.  I love it because of what happened when he landed, of course, and totally not because his name makes my inner 13-year old laugh.  https://en.wikipedia.org/wiki/No_Kum-sok

The problem is getting the aircraft into the air. If you are a defecting pilot, you have access to the fighter and a team of people who will help you get it into the air.

I understand your quibble @EricHauenstein. By flying I am including the navigation, communication and systems management required to get anywhere and land safely. I've edited to make that more clear.

I think a very important follow up question would be, "I've stolen a fighter plane. How do I stay alive and not get shot down immediately?"

@BruceWayne The follow up answer of course being "I'm a fighter pilot; I have been trained thousands of hours to avoid being shot down and escape any radar."

@Kevin That's a great story. My favorite part is that a) they tried to give it back and b) North Korea wouldn't claim it. The sad part is that his friends and remaining family were executed

"Military aircraft rarely have any sort of keys or any sort of authentication systems" In that case I believe the insurance won't work. It's contributory negligence.

@BruceWayne Such a question would immediately solicit comments such as "Why are you [I hope you're not] posting your question here instead of flying the plane?"

Perhaps you don't need to fly it to steal it. If you can get past all the other physical defenses, can you get it on a flatbed and drive off with it?

@kojiro They don't look like it, but fighters are actually VERY big. The F18 is 56 feet long with a 40 foot wingspan. The biggest 18-wheelers are 7o feet long and 20 feet across. They're about the same height. Yes, fighter planes are about as big as a semi trailer, but twice as wide.

@GdD: How would the navigation be significantly different from a light aircraft, once the higher speed is taken into account?  I'd assume you don't particularly want to communicate - in fact, may want to avoid communication (and detection).

Key don't use keys because there is no point. It takes a lot to get one of these things ready. They don't usually just sit around, and if they are just sitting around then you don't want a fast take off slowed down by "I forgot my keys"

@jamesqf - in a very real way there not much different. Pitch, yaw, thrust, and roll all kep you up and pointed in the right direction. But EVERY aircraft - civilian or military has it's own "things" that need to be done. Here is an example for a 747 http://www.onebag.com/popups/747checklist.pdf Keep in mind that some jets take a "huffer" to start their engines. https://www.youtube.com/watch?v=V0bPW3mV0RE Try using the huffer cart, and the jet and wearing those pants.

What is largely different is speed and maneuverability. "How fast is a 1 min turn in a F16?" (it's 1 min just in case your wondering) but can you even do a 1 min turn or is the control too sensitive. Where is the throttle in this thing https://s-media-cache-ak0.pinimg.com/736x/ce/5a/6a/ce5a6ab3aa591f7bd855dd58a1da5569.jpg And more important are you waring a G-suite or are you going to black out the first time you turn?

@coteyr: Sure, there are going to be a lot of differences in flying the plane.  I just meant the navigation part - I Follow Roads, right :-)  In any case, my ambitions never extended to jets, I just wanted an OV-10.

The question explicitly states someone with the required skills, so all boils down to physical access and depending on the type of aircraft support needed to start it up externally (so simply steal some aircraft where you don't need anyone external)

There's a lovely fictional example of this, grounded firmly in reality, in Adam Hall's The Sinkiang Executive (1978) where the protagonist relies on a combination of surprise and the usually-authoritarian treatment of the Soviet Union ground crew to bully them into readying a fighter for his escape. He also flies into the mission in a defector's (fictional) MiG 28D and uses bluff, rank, and an allegedly-defective radio to survive for long enough to penetrate Soviet airspace.

Lol, it was my childhood dream, to steal a jet fighter...

I flew combat attack jets and would say that taking off is challenging until one gets use to the aircraft's characteristics. For example, on takeoff the acceleration is quite quick and it is easy to end up "behind the aircraft," and sitting as it were on the end of the runway. This has implications near rotation and once airborne.  Once airborne they would be relatively easy to maintain straight and level if you had some training.

The OP is really asking a very specific question, do modern fighterds have keys?  (Or some similar system - a security pinpad, perhaps a $3 fingerprint scanner from an iPhone, or the like.)  Indeed, the OP even very specifically pointed out they are already aware of all the obvious difficulties of "hopping in and taking off".

@Fattie That's after some edits that changed the original question quite a bit. The original was little more than "what prevents me from stealing a military jet?" Changing the question in this manner is generally considered rude, but this answer still answers the question as it is now - "Nope." The extra information is still valuable, since it shows why there's no point to having keys.

@LUann - understood, good point.  Don't you hate changing question targets?  (Must be like a runway that slides around or something :) )  (Actually I guess it's like a carrier landing for you pilot folks!  :) )

I was a pilot attached to Attack Squadron 86 flying A7-E's during the mid-1980's and here is a case where one of our jets was nearly stolen. It is not what you might guess, and as GdD pointed out above, maintenance support nabbed the pilot. To this day I am amazed at the guy's audacity. Don't know what happened to him after this encounter. I was the Officer of the Day when this event occurred.
Our squadron was going through qualifications and we were responsible for providing the aircraft for the referee/spotters on a particular flight. The referee showed up from down the field and checked himself in with maintenance, going through the Aircraft Discrepancy Book. He went out to the aircraft on the apron and hopped in after his preflight.
Shortly after he left maintenance another pilot came scrambling in all suited up and ready to go. He told the maintenance chief to give him a plane, and our chief obliged not knowing who the heck he was. After going through the discrepancies the pilot headed out to our aircraft and the Chief gave me a call.
I went down to the flight line and found the pilot getting ready to climb into the cockpit after doing a kick the tires, light the fires pre-flight. I was a bit surprised to see a guy I had come through Replacement Air Group (RAG) training with. I asked him who he was flying with, and he said he was part of the flight going out. "You are not listed as one of the referees Bob." He came back, "Oh, yeah, I'm not. I was just gonna catch a flight out in one of your birds."
This is like walking up to your neighbor's house, the neighbor you never talk to, grabbing the keys to his Ferrari and taking it out for a test spin. "Hmm, no, I don't think so. Go home Bob."

go home bob. you're drunk. don't just go flying people's aircraft around.

Can a fighter jet be stolen?
Absolutely, no key required. There was the case of a lance corporal who took a USMC  A-4 Skyhawk for a joyride in 1986. See: Marine Mechanic Stole An A-4M-Skyhawk.
21-year-old Lance Corporal Howard A. Foote Jr. went for a joyride after learning he can no longer pursue a flying career in the military. He was only experienced in gliders.

A load of charges were brought against the Lance Corporal, including misappropriating the truck he used to get to the aircraft and the Skyhawk itself, as well as damaging an aircraft and disobeying regulations. He was also charged with hazarding a vessel, flying without proper training or approval and recklessly disregarding the plane’s mechanical condition at the time of flight.
Many of these charges were dropped, but Foote still could have faced nine years of hard labor, forfeiture of all pay, demotion to private and a dishonorable discharge.

Could someone steal a jet fighter? Sure.  But as mentioned above there are a few obstacles to doing this.
First you're going to have to get to a jet fighter which means you're going to need access to the flight line of a secure military facility and have to pass a number of sentries and obstacles guarded by big mean men with guns who are authorized, if necessary, to shoot trespassers; you can't just walk off the street and onto a flight line without garnering attention and apprehension by the authorities.
If somehow you could get through the defenses of such an installation and get access to a jet, you're going to have to suit up in the proper life support equipment or there's a risk the jet itself could kill you during operations, vis a vis hypoxia, G induced loss of consciousness followed by CFIT, etc.  It also has on-board equipment like pyrotechnic charges for canopy jettison and rocket powered ejection seats which can kill if mishandled by a neophyte.  You're going to have to have a good understanding of this equipment, how to connect and use it in the jet and how to handle various emergencies involving it.
After that, you're going to have to make your way onto a line with jets which are properly maintained in a flight ready condition, pre-flighted, and fueled.  The latter is rarely done unless the aircraft is scheduled for a flight.  Failure to properly pre-flight the aircraft will probably result in mechanical failure and departure from controlled flight.  An armed jet with ordnance on the wing/weapons bay stations or ammunition in the gun magazine is even rarer.  Such jets do exist but are usually kept in even more sterile and secure places places e.g. HAS, etc.
You will also have to know how to operate the thing.  As mentioned above, high speed flight in a high performance aircraft would probably saturate and overwhelm the average Joe of Jane on the street, leading to a catastrophic accident.  Sometimes jets require external start equipment and additional personnel to power up, so this might not be a one man job.  Ironically combat aircraft are fairly easy to start - if you know what you're doing and what you're looking for.
And starting up a jet will attract a lot of negative attention; again the big mean men with the M-16s aren't going to casually sit back and let you taxi the jet from the parking apron to a runway.
And even if you did get the thing airborne, where are you going to go?  You only have a finite amount of fuel on board and that jet goes through it pretty quickly.  You have, at best, about 1000 NM of range before you run out of gas and into the arms of the law on some very serious charges.  You're also likely to be intercepted by armed jet fighters flown by trained and experienced fighter pilots who will either motivate you to return the aircraft or shoot you down if they or their commanders feel you pose an immediate threat.
As mentioned above, aircraft theft of that nature usually is an inside job such as an existing military officer and appropriately rated pilot stealing a jet in order to defect to another country.  There have been a few unauthorized appropriations of a military jet in the past such as this USAF mechanic taking an F-86 on an inadvertent solo flight after a botched high speed taxi test.  Fortunately most thieves have been dumber than a bag of rocks and totally ignorant to operations of an aircraft, so their attempts usually don't go far as this car thief's attempt to steal a life flight helicopter from a hospital went.

Anything can be stolen.
If by "steal" you mean get into a fighter and fly it off by yourself, it depends on which aircraft it is and various other factors.
Many older jets cannot be cold started by just the pilot. They require a special device called an APU to spin up the engines. You would need help to operate the APU. Newer jets can be cold started by the pilot, but there are still various obstacles. The first problem is that unless the jet is on standby, it will likely be unfueled or only be partially fueled. So, if you just picked a random hangared jet to steal, it might only have 20% of a tank or less of fuel, which would limit you to a range of a few hundred miles.
Of course, if you are stealing a jet, why not steal some fuel, too? Usually fueling a high-performance fighter is a two-man job because the hose and nozzle are wicked heavy, but I suppose if you don't mind getting a hernia, you might be able to do it by yourself.
Another problem will be getting into the plane. How are you going to get up there? Get a rollaway ladder you say? Ok, and now that you are in the cockpit, what are you going to do with the rollaway? Drive over it with the aircraft? Nice move top gun.
Once you are in there, you are good to go. Military jets do not have keys. (Even most civilian aircraft have pretty rudimentary locks. Old Cessnas have ignition locks that can be picked by an 8-year-old with a bobby pin. Also, the panel is just a sheet of plastic, so you can always just reach behind the panel and connect the wires.)
Why do you ask? Planning on sneaking on to the local air base?

I won't get into specifics, but newer aircraft are taking this into consideration and employing authentication/authorization features. This could be as simple as a key you turn like in your car or as complicated as your helmet performing a retinal/iris scan before allowing the engines to ignite, weapons to arm, etc.
The other benefit of these measures is that preferences can be saved and it could allow a pilot to take their preferences with them from plane to plane.
So, older planes are totally steal-able (Discovery or someone actually has an airplane repo show where they kind of do just that), but newer planes will be more secure and "thieves" (or repo men or whoever) will need various "hot wire" like techniques to get around the security.

On the other hand, older fighter airplane models might be too difficult to fly for a pilot who does not have first-hand experience on that model. An extreme case would be a Lockheed SR-71 ("Blackbird"). Lockheed eventually replaced the analogue computers with digital ones which could better cope with abrupt input on the yoke. Still, the pilot could accidentally cause the engine to shut down. The amount of catalyst to restart an engine in flight was limited. Of course, the SR-71 had other issues which could prevent a successful theft: one is that it had to refuel in flight after takeoff.

@Klaws The SR-71 isn't a fighter.

@DavidRicherby: They did make a fighter variant. The US military didn't buy it. NASA bought up the prototypes.

These days busses and trucks have security features, so that even a driver cannot just take a vehicle. I'm amazed, that this has not been the case for jet fighters. I'm also amazed, that the fighters don't have locks, as this is a measure often employed for small planes.

A co-worker was base commander at Beale and told of a U-2 that someone tried to take when it was off-base after a diversion. He comments that aside from ground support issues, there are no real security measures on that aircraft and on the SR-71.  He doesn't have first hand knowledge of the broad classification of "fighter" aircraft as he only flew a few of them.

Never seen evidence of authentication or other security systems for fighters.  I’d need more specifics.

Once, in 1997, an A-10 pilot 'stole' the aircraft he was flying, (broke off from his flight and his assigned training mission), and disappeared from ground radar. His wrecked aircraft was later found in the side of a mountain in Colorado.  It was assumed that he committed suicide.
See this article for details.
In order to 'steal' a military jet, you would have to be proficient enough in that type of aircraft to safely take off and fly the aircraft. So, the pilots who would be capable of doing this are necessarily limited to a small number of people, most of whom are military trained pilots.

I would disagree with your statement that there is a small number of people who can "steal" a military plane. There are many people, some pilots, some non-military pilots, and lots of maintenance people who would be able to operate a plane.  I will give you credit on the A-10 incident; good point.

Well, I guess that depends on how you would define small. It's certainly "small" when compared to the number of people currently alive, and still "small",  imho, compared to the number of pilots alive, and, probably, still less than 10% (is that "small"? compared to the number of living military pilots (at least not without some training in-type).

To steal an aircraft, one does not need training in-type. If one is smart enough to be flight trained, one is smart enough to find open source operational information. Besides the procedures on many aircraft are not radically different.  I never ever operated a tank, yet with about 30 minutes training, I was running an untrained crew (with 30 minutes also) on a tank simulator at Ft. Knox.  I have no time in fighter aircraft, yet after a 5 minute briefing I was taking off and doing maneuvers in a full motion simulator. Give me enough motivation, and who knows what might be possible!

@monge,  If you think that simply knowing the procedures is sufficient to operate a high performance jet aircraft safely, you are wrong. Take a physicist with total knowledge of the physics, aerodynamics and structural knowledge of what happens when a man swings a bat to hit a baseball, and stand him up with a bat in front of a major league pitcher.   And your simulator analogy does not hold water. Ask any military pilot qualified on a high performance aircraft that uses simulators what happened the first time he/she attempted to land one. With experience in other jets, sure, but a novice. no.

I respectfully disagree with your comment. My experience and that of my colleagues is such that with exposure to the aircraft, knowing the procedures and having familiarity would be enough to operate most aircraft adequately to "steal" one.  The theft does not exercise the full gamut of aircraft performance, and many of the operations have analogs with similar aircraft. Sure, there are exceptions which are operational today, like the U-2 and the AV-8B. And if one were performing arrested landings or assisted takeoffs there might be issues.

Well, also respectfully, I guess I am saying is that this would depend on the nature of the "exposure" to the aircraft an individual has experienced. If this exposure is only of an academic, intellectual nature, and not kinesthetic knowledge, then you would fail,

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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Do modern airliners have a flaps warning indication?
Years ago I was on a flight from San Francisco to Los Angeles, a 737 if I remember rightly, and was sitting just behind the wings and could not help noticing the flaps were partly extended for the whole flight.
I thought they looked odd out the window but did not bother mentioning it to the cabin crew since I was not sure what I was seeing. However, on landing in LA, the pilot retracted the flaps after using them for landing, and they were then correctly seated.
I assume the pilot felt something was off, since after landing I could see ground crew wandering around and inspecting the aircraft with puzzled looks on their faces.
I understand that the flap controls are a bit out of the pilots normal sight lines and this got me wondering whether there is instrumentation in the cockpit that alerts the pilot to the fact that the flaps are not fully closed once in normal cruise attitude. Perhaps even an indicator similar to the landing gear lights.

There is a position indicator that shows what position the flaps are in.  If they are in something other than "up", then the green "LE Flaps Ext" light would also be illuminated.
Photo of 737 Flap Position Indicator
That's not to say that those indications might not be missed.
There are a couple of other things that would be not-quite-normal (i.e. VNAV information would be unusual, commanding a speed no faster than the speed limit for the current flap configuration), but if you're missing the one indication, you might well misinterpret the others.
That said, there is no aural warning for the flaps being oversped the way that there is for an MMO or VMO overspeed (the clacker) or an approach to stall (the stick shaker).  The only warning per se would be on the speed tape on a PFD or EFIS display or in the HUD.  And in a Classic (non-EFIS) 737 without the HUD in use, those wouldn't be seen.
The most obvious indications would be the leading edge flaps and leading edge slats rattling around, and the hit on performance during climb and cruise (the leading edge devices are a lot of drag).  Generally that sort of thing is pretty hard to miss, but again, if you're having the kind of day where you're missing the call to retract the flaps and missing the position indicator & its green light, you might miss the others too.
My first reaction on reading that account was, wow that crew messed up big!  On the other hand, the "quiet wing" mod mentioned in the other answer makes very good sense as an explanation -- especially if the OP does NOT remember the buffet from the leading edge devices banging around.  Let's go with that one!
By 2005, I'd hope that nobody was pulling CB's in order to extend the trailing edge flaps only -- the cautionary tale of what happened to the 727 whose crew did that was pretty well known by then.  (Essentially, somebody who wasn't "in" on the plan saw the CB out & pushed it in, extending the leading edge devices at high speed. One of them departed the aircraft, and the it took tens of thousands of feet, IIRC, to regain control of the aircraft. Bad scene!)

"the aircraft spiraled out of control, diving about 34,000 feet (10,000 m) in just 63 seconds" - Yikes! Good thing they were cruising at 39,000, I guess. 34,000 ft in just over a minute is quite the descent rate.

That's a very significant fraction of cruising speed...straight down!  Are you sure that's correct?

@AaronD Since "out of control" can mean "you just stopped flying and the plane is now a falling brick" that descent rate isn't impossible. If you happen to be falling nose first, the engines are also helping you hit the ground sooner, of course.

@alephzero  I can believe that that speed could be reached, but I'm thinking also about the implied recovery.  (or maybe I'm combining two incidents)  What kind of acceleration does it take to cover that distance in that time, being vertically stationary at both ends of the ordeal?  And is it survivable?

@AaronD - assuming equal acceleration and deceleration, that means 5000m in 31.5s in each half of the movement. With zero initial vertical motion, that means acceleration is 2*5000/31.5/31.5 = about 10m/s/s, or roughly one g.  Uncomfortable, but definitely survivable.  Looked at another way, this is basically the trajectory the vomit comet follows, albeit maintained for a little longer time than usual.

Well when I think back it was probably in the early 80's this occurred so the 727 hack is quite possible. I could not see any lead edge change and the flight was smooth enough. I definitely would have mentioned it to someone had those effects been evident.

Thanks for the picture though.. Other answer was not clear in that respect. WIsh I could accept both of them.


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Do pilots need a fuel pressure/temperature gauge?
Fuel pressure gauge allows you to monitor the fuel pressure while the engine is under a load. However, since the fuel pressure requirement is listed by the engine manufacturer, do pilots even need a gauge to see that the fuel pressure is within normal range? Could we remove the gauge and just warns the pilot if the fuel pressure is too high or too low? And why there isn't a fuel temperature gauge?
Edit: I am thinking about a small, four to five passenger civilian aircraft for urban mobility

Replacing a pressure gauge with a fault indicator gives the pilot less information; it’s hard to see how that is an improvement.  In the case of a failing fuel pump (for example) the pilot would have advance warning before the pressure was out of spec.

Much depends on your specific aircraft.  Some have a need for fuel pressure values, others don't.  Likewise fuel temperature readings.
On the 737, there is a light as you describe next to each fuel pump switch, linked to a sensor just downstream of the corresponding fuel pump.  If the light's on, the pressure is low and the pump should be turned off because it has failed.  I don't think Boeing even tells us what the pressure is in PSI; they just say "If the pump output pressure falls below a limit value then...".  I can't read the PSI, and I can only affect it by turning the pump on or off... so all I need to know is "sufficient" or "not sufficient."  That's it -- no value to be gained in showing me a more exact number, nor in having me memorize its limits.
The 737 also has a fuel temperature gauge, which reads from one tank -- with the assumption that this is representative of the others.  My guess is that Boeing has done enough testing that they're happy with that assumption.  If that temperature gets cold enough (rare, but possible), then you have to do things to increase that temperature -- go faster or go lower, or both, generally.  In this case, there is enough value in seeing that trend (plus, knowing the number in case of getting cold-soaked fuel frost or other icing on the wing) that we DO get a numeric value.
Photo, from http://www.b737.org.uk/fuel.htm:

The fuel temperature gauge is circular, directly above the blue (crossfeed) VALVE OPEN light in the picture.
Note that all lights are on, so this was taken during a "lights test" -- in normal operation, the low pressure amber lights would all be extinguished. The logic for them is that the main tank switches are on when the engines are running, and an amber "LOW PRESSURE" light is telling you that the pump isn't running (which is either a failed pump, or an electrical bus unpowered, or the pump switch turned off, or some rare conditions like a popped circuit breaker or failed sensor).  The center tank amber lights are active when the switches are on, but are off when the corresponding switch is off.  This way, when you aren't using the center tank, you don't have the amber lights on continuously; it's normal to get the center tank LOW PRESSURE lights coming on when you exhaust the center tank, which is normal ops.  Then you turn off both switches, and both lights are out.
The case where you'd turn off a not-failed main tank switch in flight would be to cross-feed fuel (i.e. feed both engines off of one tank) in order to balance the main tanks.  In that case, the amber lights for the turned-off pumps are ON, because you're in a condition that you will want to correct (after the fuel is balanced).
Of course, sitting at the gate with engines not running & all pumps turned off, you'd have the 4 main tank LOW PRESSURE lights illuminated, but in that condition you have plenty of amber lights lit up on the overhead panel, and that's normal for being at the gate & not running.

If your fuel system has pumps, a pressure gauge is important because it provides a precise status of the pump and can provide early warning of future problems, like pressure fluctuations, or declining but still acceptable pressure.
A warning light is there to get the pilot's attention when things really go south.  In an ideal world, you'd want both, but of the two, the gauge is more important and if forced to choose one or the other, it would be the gauge.
My own plane has a pressure gauge but no low pressure warning light, and that is more than sufficient because the airplane is carbureted and the actual fuel pressure requirement is very low, a fraction of a PSI, and the system runs at about 5-6 PSI.  It would be nice to have a low pressure warning light, but I don't miss it because I have a good instrument scan and problems are easy to spot.
If you have fuel injection, the pressure requirements are much higher and the gauge has a more critical function because there is less margin for degradation before unpleasant difficulties arise.
Fuel temperature is not important if you are running gasoline, so temp gauges are pretty rare with piston engines.  But for kerosene, being a relative of diesel with viscosity issues when cold, temperature becomes important once it gets really cold (jets generally require fuel heaters and will have bulk fuel temperature limits that have to be observed) so fuel temperature indication is a basic part of the fuel system.

Good example of the other side of the coin from the 737. With different systems & operating conditions, a completely different set of indicators makes better sense in this application.

Regarding small aircraft, I had an experience this morning that shows why a pressure gauge is useful.
On the right tank, I was in the center of the green arc without the boost pump. On the left tank, though, it was at the low end of the green arc. Technically still okay, but I felt safer leaving the boost pump on when using that tank. More importantly, I know there is something on that side that needs fixing before it becomes a critical problem in flight.
I’ve never worried about fuel temperature; avgas freezes at -100°C, and my cabin heat is nowhere near good enough for me to fly if it’s that cold. However, jet fuel can freeze at up to -40°C, which jets can easily encounter at high altitudes, so that’s an issue they do have to worry about.

If you have a carb, it only needs about a quarter psi to get enough flow to keep the bowl full.  In fact if the fuel tank is not much lower than the carb (like in a Cherokee) if you had a double pump failure you can probably get sufficient flow by skidding hard away from the in-use tank.  My PL-2 has tip tanks that are somewhat higher relative to the carb, about level with it, and it will gravity feed from a tank with just a little bit of skid.

@JohnK That plane is fuel-injected. Does that change your comment?

Absolutely..... fuggedaboutit lol  Wasn't it a Piper Arrow or something you were flying?

@JohnK Yes, that one was the Arrow.

We had an Arrow in our flying club in the mid 70s with the auto extension system.  Our pres told me one of the members was flying it on a descent at high speed with the engine at idle.  He unwrapped a stick of gum and pushed the foil wrapper out the little bottom hinged vent window.  The foil was caught by the speed probe used by the extension system, below the back window IIRC, and covered the dynamic pressure port, removing the speed input from the auto extension controller.  Down came the gear... Nothing was damaged.  I had heard that a lot of owners disabled it.

@JohnK Ours doesn’t have that. I asked the CFI who did my transition, and he said such systems likely cause more gear-up landings (either by failing, or by training pilots not to lower the gear in their next plane) than they’ll ever prevent. Vle is higher than max cruise, so accidental extension is rarely a concern.

I think Piper removed the system in the 70s or 80s after too many complaints about problems.  I did a flight or two in the club airplane, which was from around 1970, but never got checked out in it.


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What are service letters and service bulletins?
How do a service letter and service bulletin differ.

Related question and answer: https://aviation.stackexchange.com/q/43580/8079

Possible duplicate of What are the following documents about aircraft maintenance?

"Depending upon the manufacturer, a service bulletin may also be called a "mandatory service bulletin," "technical service bulletin," "service letter" or "service instructions." Service bulletins are automatically sent to the owner of an aircraft by the aircraft or component manufacturer. However, aircraft owner's should be aware that service bulletins are not automatically sent to maintenance providers." https://blog.globalair.com/post/Service-Bulletin-What-is-an-Aircraft-Owner-to-Do.aspx

In the United States and presumably in other countries, there is no Government oversight on how a manufacture communicates to aircraft owner/operators except that they must issue some form of mandatory compliance "Service Bulletin" for Part 135/121 commercial operators. There is no industry guideline for what a document is called, or the information it conveys.
Manufactures often use an assortment of documents and the type of information in each document is left up to the company. For example, Continental Motors uses almost every kind of document I have ever seen...

"Service Bulletin" i.e. SB "used by most manufactures..."
"Critical Service Bulletin" i.e CSB 01-1 "Fuel Pump Inspection..."
"Service Bulletin Mandatory" i.e M64-18 "Field Conversion to
    Turbocharging..."
"Service Information" i.e. SIL99-2A "Current Listing of Sealents..."
"Service Information Directive" i.e. SID97-4C "Cylinder Bore And Piston Fit..."
"Service Letter" i.e 556c "Shouldered magneto drive shaft bushing..."

There are others I don't have copies of such as information sent to marketing, promotions, or financial departments. Additionally, the FAA will require specific forms to be used as part of the approval for Part 135, 119, and 121 operating rules. For example some type of "trace-ability" paperwork is required to be attached to all materials and parts. A "trace-ability" document provides manufacture, model, mil-spec, fire resistance, etc.
It is interesting to note that despite the universal use of the Red, Yellow, and Green parts tagging system, their is no FAR regulation requiring it's use.

It's usually a matter of priority and importance. A Service Letter is normally used to pass information along to the industry from the manufacturer and are not mandatory. Service Bulletins are highlighting more important service difficulties found in the field. They can be labelled as Mandatory by the manufacturer, but still are not required to be performed on privately owned aircraft. Commercial operators are normally required to comply with mandatory SB's by the regulatory authority (FAA, TC, EASA, etc.). Mandatory SB's are often the basis of Airworthiness Directives published by the regulatory authority.
If you want to check them out for yourself to get an idea of how they differ, check out Piper's technical documentation repository here:
http://www.piper.com/technical-publications-documents/


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How does a G1000 equipped with a GDC 74 calculate IAS from dynamic pressure?
How exactly does a G1000 system equipped with a GDC 74 air data computer calculate the indicated airspeed that is displayed on the PFD from difference between the total and static pressure (i.e. dynamic pressure) measured from the pitot-static system?
The reason I ask is because I want to reverse the calculation and convert KIAS back to dynamic pressure, i.e. $ q $ as defined below:
$$ q = \frac{\rho u^2}{2} $$
My objective is to calculate dynamic pressure from the KIAS value that is logged to the data logs written to the top SD card by the MFD. The problem is that there are multiple ways to perform this calculation -- i.e. does the G1000 display an indicated airspeed that is uncorrected for instrument and position error (IAS, in the traditional sense) or corrected for instrument and position error (CAS)?

The ADC uses pressure transducers inside the unit to convert the pitot and static pressures to electrical voltages that are digitized and processed by software.   How you would tap into the raw signal, dunno.  There might be a ground maintenance function in the unit that allows the real time raw signal data to be displayed.

I am trying to calculate dynamic pressure after the fact from the KIAS value that is logged once per second to the G1000 data logs.

I'm not sure if it is the same for all G1000 installations, but according to this Integrated Flight Deck Pilot's Guide the flight data logging feature records both IAS in knots and TAS in knots.
Do you have the Pilot's Guide for your aircraft? It would likely indicate the values saved in there as well.

Yes, but my concern is that it is not unheard of for an indicated airspeed that is corrected for instrument and position error (CAS) displayed on an EFIS to be referred to as IAS.

I just realized that there was a really easy way to check this all along... just look in the POH and see if the prominent airspeed limitations such as Vne/Vmo are shown as IAS or CAS on the airspeed tape. They are shown in IAS on the airplane that I am interested in, so you are correct. Thanks!

Quite true. If it is corrected for position error, how would you propose to remove the position error error correction from the indicated airspeed?

For every certified airplane I know of there is a table in the POH with correction factors for converting between IAS and CAS. I wanted to be 100% sure that I was starting with IAS or else it would throw off the dynamic pressure calcuation.


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Questions regarding VTOL and VMU inhibits
I have been working on a game which has futuristic VTOL-capable air and aerospace crafts.
A player recently wrote that the stall warning (tone and aural) and AoA (tone and visual) were distracting/annoying/unnecessary/spurious during VTOL takeoff and landing. Same with the "don't sink" EGPWS call-out.
EDIT so that the post is less opinion-based?:
How should these be inhibited for helicopter or VTOL operations without sacrificing safety? For example, if the IAS is less than 50 knots we can assume that they are not trying to fly or recover and therefore we can ignore the AoA information? Or similarly if the AoA is more than 45 degrees up or down? That coupled with the gear being down or flaps in landing position? I also have other air data, radar altimetry, etc that could be used.
The game is called Rise: The Vieneo Province if you want to check it out!

The concept of stall and the associated AoA limits aren't meaningful for a VTOL aircraft in the way you've described.  Those systems are designed to prevent a pilot from accidentally stalling the wing; but during the vertical flight phase lift is not being generated by a wing but some sort of propulsion system. So stall warning would be meaningless and likely distracting.
It might be better to look at the alerts that exist for helicopter pilots. Things like low rotor RPM alerts could translate more directly to your scenario (although that depends on how far in the future you're thinking).
A system like EGPWS or HTAWS, which is the helicopter variant, would be the best place to start for designing your alerts. There are specific rulesfor how those systems need to be designed if you want to really get into it.


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what is the mechanism of hydro-mechanical system in power control unit that uses follow up feedback?
I read somewhere that the follow up system aids in actuator deflection in specified position by controlling hydraulic fluid to the actuator , but how?

what aircraft? I'm not sure that I can answer your question but it would help to have more details on what you are actually asking

on aircrafts  equipped with half powered flying control systems which utilized hydro-mechanical mechanism, also maybe nowadays is obsolete. sorry i can't mention a specific type!

Are you asking for the principle of a PCU in general? If so see this, else clarify your question, maybe also providing the link to the source.

The question seems to be on how hydraulic power actuation in aircraft works.

There is a position control loop in there that opens the servo valve until the required position is reached, then closes it. The loop uses position of the actuator as a feedback signal, measured by a position transducer, usually an LVDT. Open loop servo valves just generate an actuator velocity, not very desirable when flying a plane.
A more robust system used on aircraft has the hydraulic actuator moving together with the flight control surface. The pilot control stick is connected to the servo valve, and when he deflects the stick he opens the valve and oil starts to flow. This makes the flight control surface deflect. The actuator moves with it, in doing so closing the valve again.
You mention half powered controls in a comment - I assume you mean boosted flight controls, where the pilot directly deflects the control surface but hydraulics help in overcoming the aerodynamic forces. These work in the same way as described above, with a follow up mechanism that moves the actuator until the valve closes, but the mechanical linkage is different.
Here is a schematic diagram of a hydraulic actuation system with a mechanical servo valve:

You can see that the cylinder is anchored to the airplane structure. The diagram is a bit primitive, but the gist of it is:

The elevator is at rest, not moving.
The pilot gives a control input, which opens the servo valve (the little box on top of the cylinder).
The piston deflects because the servo valve is open.
The pilot input linkage is attached to the piston as well. Because of the movement of the piston, the servo valve closes again.

This mechanical feedback system is robust because there are very few electronics that can fail. In this set-up, elevator deflection velocity is proportional to stick deflection. Other transfer functions are possible bit programmed feedback loops and transducers.
This system is fully irreversible: airforces on the elevator push against the cylinder piston and are not fed back to the mechanical linkage at all.

i appreciate for your thorough and clear answer!

@Mehdishelby If you edit your question it will be put on the stack for reopening by the reviewers, I'll then vote to re-open it.


text:
While flying, how do I contact someone who is on the ground?
As a regular General Aviation pilot, how do I contact or relay a simple message to someone on the ground? For example, to update a friend, family member, or taxi company with an ETA or a meeting point. Assume that asking them to follow a tail number on something like FlightAware would not suffice.
Cell phones are not an option. FAA AC 91-21.1B prohibits cellular communication while in flight (Correction: see comment below; it's really the FCC that prohibits cell use), and cell reception above 5000ft AGL is pretty poor.
Commercial operators have access to more sophisticated services like ACARS, ARINC, or satellite communications, but these are usually too expensive or complicated for the regular GA pilot.
Can Flight Service Stations (FSS) call someone and relay a short message for me? Has anyone tried this?

Really before you take-off you should know when you are going to land +/- a short period. As a pilot, putting the aircraft up takes quite a bit of time, so after landing I'll call somebody and even if it takes them 15-30 minutes to get to the airport, I'm not waiting long.

BTW, 91.21 only applies to aircraft flying IFR or if your aircraft is operated by the holder of an air carrier certificate. It is not prohibited (by the FAA) for use by GA pilots in VFR conditions. Many GA headsets contain a Bluetooth link to cell phones, and some COM panels also have bluetooth cell-phone interfaces, my aircraft has one.

@RonBeyer But according to Advisory Circular 91-21.1B (linked to in the question), "The Federal Communications Commission (FCC) currently prohibits the use of cell phones while airborne." See also: https://aviation.stackexchange.com/questions/1998/if-the-radio-fails-can-a-pilot-contact-the-tower-by-cell-phone

@TannerSwett Yes, which is why I said "It is not prohibited (by the FAA)". The FCC bans this and the "FAA supports it", but the FAA does not ban it. FAA AC 91-21.1B does not prohibit cellphone in flight (as described in the question). The FCC has not taken any action in the 30+ years regarding this rule, and many pilots have used cell phones in emergencies without any word from the FCC. I'm not advocating its use in non-emergency situations.

Thanks for the correction, @RonBeyer. But regardless of the legality, I've found that cell signal is pretty unreliable above ~5000 AGL, so even a Bluetooth-linked headset won't cut it.
And yeah, I ought to know the ETA, but it'd be useful to send other types of messages, like an unplanned diversion.

One could always fly at less than 5000 ft AGL, at least for as long as needed to place a call.

We carry a portable Sat phone with us that we move between our plane and helicopter. The upfront cost is not that high compared to the peace of mind it provides.

Buy a Bluetooth headset.

Once, in a pinch, I persuaded Phoenix Approach to call my mother-in-law and advise her to meet me at an alternate airport since I would have had to hold for two hours to get into PHX where we'd originally planned to meet. This is definitely not something that ATC does on a regular basis.
Other options include carrying an Iridium handheld satellite phone (not cheap, but certainly a possibility for light GA) or installing an amateur radio transceiver.

Would CB radio be an option if licensing is problematic?

One option is the Spot Beacon, which sends pre-prepared Text messages and location updates via satellite to an SMS phone number and updates are visible on a website.

If you are properly licensed (and it’s not all that hard to get licensed) you can use a ham radio to talk to people on the ground from a GA aircraft. This provides a relatively inexpensive option when compared to something like a satellite phone.